Many industrial molded case circuit breakers have terminals and provisions for wiring connectors at each end of the circuit breaker. Frequently in the design of circuit breakers, a removable terminal cover is provided. With the terminal cover removed, the electrician has easy access to the breaker terminals for assembling, tightening, or inspecting the electrical connections. When the terminal cover is replaced, the cover helps prevent accidental contact with the live terminals from the front of the circuit breaker. A second function of a terminal cover is to prevent hot, ionized gases caused by short circuit interruption from exiting from the front of the circuit breaker. Such gases could strike grounded metal on the front of the equipment enclosure and cause an electrical ground fault.
Typically, hot, ionized gases from short circuit interruption are exhausted from an arc chamber of the circuit breaker. Often, the design of the circuit breaker is such that the hot gases flow against the terminal connections before exiting the circuit breaker. After the gases exit the circuit breaker, the exhaust from the different poles of the breaker, which are at different electrical potentials, mix together. This mixing can result in a phase-to-phase arcing fault across the terminals of the circuit breaker. Additionally, there exists the potential for shorting the electrical path through the circuit breaker arc stack by arcing from the open moving blade directly to the lug, thereby eliminating the arc stack from the current path. This may result in the circuit breaker being unable to perform its primary function of interrupting the short circuit current.
The problem of phase-to-phase arcing in a circuit breaker has been addressed. One solution is to lengthen the walls of the molded case so that the walls extend further beyond the wiring lugs. This lengthens the path required for a phase-to-phase arc and thus reduces the likelihood that a phase-to phase arc will occur. However, this solution has the disadvantage of increasing the overall size of the circuit breaker.
Another solution is to guide the gases so that they exit the breaker without coming into contact with the breaker terminals and wiring lugs. U.S. Pat. No. 5,811,749 provides a terminal cover having chamber vents. Access holes for receiving a tool are provided to reach through the cover to tighten the terminal screws. The access holes are tubular in shape and extend through the arc chamber vents. Thus, the access holes disadvantageously reduce the cross-sectional area of the vents and restrict the flow of gases. In addition, the tubular access holes require a complicated and costly mold which requires cross-slides from opposite directions.
Accordingly, there is a need to provide an improved terminal cover for a circuit breaker which effectively permits arc gases to be vented from the circuit breaker while preventing access to the line terminals.